MySql 如何合并一列中的两列，使第二列输出在多行中位于第一列

Question

How to Merge two columns in one column Such that second column ouput goes uder first column in multiple rows without using UNION in single query.

for Eg:

column1 column2 
1          3
2          4

expected OP:

 column1 
      1
      2
      3
      4

Answer 1

without using UNION in single query

SELECT DISTINCT CASE WHEN t00.id = t01.id
                     THEN t1.column1
                     ELSE t2.column2 
                     END all_values
FROM       ( SELECT column1 id 
             FROM test
             ORDER BY 1
             LIMIT 2 ) t00
CROSS JOIN ( SELECT MIN(column1) id
             FROM test ) t01
CROSS JOIN test t1
CROSS JOIN test t2
ORDER BY 1

fiddle

The table must have at least 2 rows.

Applicable for 5.x MySQL version.

Answer 2

union all is the right approach:

select column1 as col
from t
union all
select column2 
from t
order by col;

You can also use a cross join with the union all in a subquery:

select (case when seqnum = 1 then column1 else column2 end) as col
from t cross join
     (select row_number() over () as seqnum
      from t
      limit 2
     ) tt
order by col;